Resistor



,1 1- H. H. WILLMANN 8, 0

RESISTOR File d Feb. 12, 1938 WITNESSES: INVENTOR 458M; Hm H. mum/m.

QU/QW- A i 2' 4 v A ORN Patented Jan. 1, 1941 UNITED STATES PATENTOFFICE aasrs'roa Application February 12, 1938, Serial No. 190,271 InGermany February 13, 1937 3 Claims.

My invention relates generally to resistors, and it refers particularlyto an improved form of construction for facilitating the cooling ofresistor units.

Where resistors are used, some-provision must necessarily be made toallow for the dissipation of the heat generated therein. Usually this isprovided for by uslnga conductor of sumcient cross section to provideadequate conductor heat radiating surface. Where weight and space arenot important factors, such provision may be allowable, but as the crosssection and weight of a conductor increases as the square oftheconductor diameter and the surface area of .the conductor merelyincreases in direct proportion to the diameter, it may be seen that inproviding sufilcient conductor surface area for the cooling of theconductor theweight-of the conductor per unit of length may in someinstances be greatly increased. Furthermore, as the resistance of theconductor is lowered with increase of conductor cross section, the useof a larger conductor to provide sufficient cooling surface, will at thesame time necessitate the use of a greater length of conductor in orderto provide a given resistance. Thus the weight and size of the resistoris greatly increased where such provision for cooling is used. Y

Where weight and space are important factors, it will usually be founddesirable to utilize a conductor member having a relatively small crosssection so asto operate the conductor at its most efficient currentdensity. In order to provide for the cooling of the conductor memberunder such circumstances, it is desirable to increase the effectivecooling area of the conductor without increasing the actual dimensionsof the conductor itself. In some instances this result has been attainedby the use of forced draft ventilation, which increases the effectivecooling area of the conductor by increasing the velocity of the air overthe surface of the conductor, thereby more rapidly transferring the heatfrom the conductor to the surrounding atmosphere. Such a scheme, whileeffective in maintaining the conductor at desirable operatingtemperature, nevertheless utilizes additional equipment, thereby addingto the weight and space occupied. In other in stances, attempts havebeen made to use metal supporting tubes in connection with bare resistorconductor members to increase the effective cooling surface of theconductor, a layer of enamel being provided on the surface of the metaltube to insulate the conductor member therefrom. However, it has beenfound that, in order to provide a film of enamel sufficiently thick toprovide adequate electrical insulation between the bare.

conductor member and the metal tube, the resistance to the flow of heatbetween the conductor member and the support tube is also greatlyincreased and the cooling effect of the' ber for effecting a ready flowofheat therebetween.

A further object of my invention is to utilize an oxide coated metallicsupport member for a bare resistor conductor member to facilitate thecooling of the conductor member.

Another object of my invention is .to provide an anodized aluminumsupport tube for a bare resistor conductor member, wherein] theconv,ductormember is wound in close contactwith the support tube toeffect a ready flow of heat between the conductor and the support tube.

Still another object of my invention isto provide a light weightresistor that shaJl-be ;simple and inexpensive to manufacture, anddurable in operation.

Other objects will in part be obvious, and in part appear hereinafter. v

For a complete understanding of the nature and scope of my invention,reference may be had to the following detailed description, taken inconnection with the accompanying drawing in which:

Figure l is a partially sectionalized view in side elevation of aresistor embodying the principal features of my invention, and! Fig. 2is a sectional end view of an alternative the prinbe formed thereon inany well known manner. such, for example, as by immersing the who in abath of salts of a desired composition and making the tube the anode inan electrical circuit, whereby an anodic coating may be deposited on thesurface of the tube.

It has been found from practice that if the support tube II be ofaluminum this coating, while exceedingly thin, has very desirableinsulating characteristics, and may be subjected to relatively highmechanical stresses without falling. It has further been found that bymaking the conductor groove 12 of such dimension that the conductor Mwill be relatively deeply embedded in the surface of the support tubell, 8. high degree of heat conductivity may be secured between theconductor and the supporting tube which was not possible with previousforms of support tube constructions.

In this manner the cooling surface of the supporting tube may beefficiently utilized to aid in cooling the conductor member, as byfacilitating the flow of heat between the conductor and the supportingtube, the conductor may be maintained at a temperature which is butslightly greater than the temperature of the support tube, and thesurface area of the support tube may be practically considered as anextension of the surface of the conductor, so that the conductor may beoperated at a relatively high current density without fear ofoverheating. Thus a smaller and lighter conductor member may be usedthan would otherwise be possible, and the weight and size of theresistor may be greatly reduced.

In order to provide terminal connections for the resistor, insulatingbushings I! may be provided, being for example, inserted in openings inthe tube itself in any well known manner, for accommodating terminalscrews 20. It has, however, been found that if the tube II is providedwith an anodic coating over the entire surface, terminal members may befastened directly to the metallic tube without the use of suchinsulating bushings, and thus the bushings ll may be dispensed with, ifdesired.

It has been found that by providing cooling fins on the support tubeill, the cooling effect of the support tube and hence the effectivecooling area of the conductor member l4, may be further increased. Asthe metallic support tube I. need only be relatively thin in order toprovide sufficient mechanical strength for supporting the conductor I4,I have found it advantageous to produce the conductor groove l2 by meansof a pressing operation, whereby the metal of the support tube Hi isdeformed to provide the groove I! on the outer surface and a projectingrib M on the inner surface. This form of construction facilitates incooling the support member by making provision for the imbedding of theconductor in the outer surface of the support, so as to aid in thetransfer of heat therebetween, and at the same time provides additionalradiating surface for the support tube.

Referring to Fig. 2, it may be seen that in certain instances where itis desirable to group a 'full advantage may be taken of the high heatconductivity and radiation characteristics of a metallic support memberin connection with a bare conducting member. By interposing an oxidefilm between the support and the conductor the flow of heat therebetweenis not appreciably impeded, while the electrical insulation therebyprovided is exceedingly good. Thus the resistance element may be workedat relatively high current densities without any danger of overheating,and the weight and size of the resistor may therefore be greatlyreduced. Since the mechanical qualities of metal supports are decidedlysuperior to those of the refractory or other insulating material, afurther reduction in weight may be effected by use of thinner crosssections of metallic support members without any sacrifice of eitherstrength or cooling effect.

Since certain changes may be made in the above construction anddifferent embodiments of the invention may be made without departingfrom the spirit thereof, it is intended that all the matter contained inthe above description or shown in the accompanying drawing should beconsidered as illustrative and not in a limiting sense.

I claim as my invention:

1. The combination in a resistor of a tubular metallic support memberdeformed to provide a groove on the outside surface thereof and acooling fin on the inside surface, an oxide insulating film covering atleast said outside surface, and a bare resistance member disposed in theoutside surface groove in close contact with the support member, wherebythe support member is utilized to increase the effective cooling area ofthe resistance member.

2. A resistor comprising, a hollow elongated metallic support memberhaving relatively thin side walls with a continuous helical grooveformed about the outer surface thereof so as to provide a correspondingridge on the inner surface, an insulating oxide film formed on the outersurface of the support member, and a bare resistance element positionedin the groove on the outer surface and insulated from the support memberby the oxide film.

3. In a resistor in combination, a relatively thin walled tubularaluminum support having a helical groove formed in the outer surface soas to provide a projecting ridge on the inner surface thereof, a bareresistance wire positioned in the helical groove, and an insulatingoxide film formed on' the support so as to insulate the resistance wiretherefrom.

HANS H. WILLMANN.

number of such resistors in close association, it

